This week, UK company Ultra Light Rail Partners (ULRP) made headlines when it received a government grant to pursue an interesting technology: biogas-powered trains. In an effort to increase sustainability, the company is working to develop new, lightweight trains that will run on waste materials, offering an eco-conscious alternative to fossil fuels. 

 

Concept of BioUltra

Concept of BioUltra. Image used courtesy of the Black Country Chamber of Commerce and The Independent
 

The company plans to store the converted biogas energy via mechanical storage. 

 

A Brief Review of Biogas Technology

Biogas technology can be defined as the generation of combustible gas from anaerobic biomass digestion. Sources of this biogas include animal and human biowaste, plants, and wastes from agriculture and slaughterhouses. 

The gas itself consists primarily of methane and CO2 with its constituents being 50–75% and 25–45% respectively. Since energy content is primarily determined by methane content, biogas isn’t as energy-dense as natural gas, which is roughly 80–90% methane.

 

Excrement Source 

Liter of Biogas per Kilogram of Excrement

Cattle40
Buffalo30
Pig60
Chicken70
Information used courtesy of Energypedia
 

The average calorie content of biogas is about 21–23.5 MJ/m³, meaning that 1 m³ of biogas corresponds to 0.5-0.6 L diesel fuel or 6 kWh. Being that a standard EV takes 30 kWh to drive 100 miles, you would need 5m³, or 5,000 L of biogas to support the vehicle. This would require roughly 70 kg of chicken excrement to fulfill. 

 

Electricity Generation From Biogases 

While biogas could technically be converted into electricity using a fuel cell, this method is too expensive and infeasible. Instead, most biogas is converted using generators based on combustion motors with high compression rates. This process converts chemical energy to mechanical energy, then powers an electric generator to create an AC output. 

 

Biogas plant schematic

Biogas plant schematic. Image used courtesy of Energypedia
 

According to the ULRP website, the vehicle in development will function by marrying biomethane gas-to-battery technology with flywheel power storage to minimize air pollution. Flywheel energy storage uses the aforementioned electric motor to rotate a flywheel, converting the electrical power back to be stored as mechanical power. 

 

Mechanical to Electrical to Mechanical Energy?

For electrical engineers, the proposed method of energy storage could raise some questions. One concern might involve the redundancy of converting the biogas to mechanical energy (via the combustion engine), then into electrical energy, then back into mechanical energy in the flywheel. This process may at first seem inefficient unless ULRP’s plan is to skip the electrical conversion process.

 

Flywheel storage diagram

Flywheel storage diagram. Image used courtesy of Beacon Power, LLC and the Energy Storage Association
 

However, mechanical storage is significantly more sustainable than battery storage in that they are capable of millions of full “charge and discharge” cycles without any storage degradation, meaning that they have life cycles of 15–20 years. Additionally, flywheels can have efficiencies up to 95% and are comparably insensitive to the effects of temperature. 

 

Sustainability as the Goal

Biogas-powered electronics seem like a good idea in theory since they convert natural gases (which would’ve been released into the air anyway) into fuel. 

From an EE perspective, URLP’s concept of taking biogas and storing it as mechanical energy might seem confusing at first. However, for a company that is focused solely on sustainability, a storage system that has a longer lifespan and high efficiency without toxic materials (like batteries) may be an effective idea.